1. Field of the Invention
The present invention generally relates to the field of optical wavelength splitters/combiners, more particularly it relates to fiber-to-the-premises network equipments comprising optical wavelength band splitter/combiner.
2. Description of the Related Art
Currently deployed fiber-to-the-premises (FTTP) networks incorporate gigabit passive optical network (GPON) for broadband delivery of voice, video and high-speed data directly to the home or broader community through optical fiber. Converged voice, video and data services networks are also known as “triple play networks”. These networks support two signals in downstream direction (from central station to the user) and one signal in upstream direction. A first downstream signal delivers analog television and a second downstream signal delivers digital voice and data services, such as for example telephone and/or Internet. The upstream signal is typically a digital signal delivering voice and data from the user to the central station or the service provider. Typically, FTTP system delivers voice, video and data over a PON using the ITU-T GPON standard. The system supports radio frequency (RF) analog video delivery using a 1550 nm wavelength overlay. High-quality video on a PON is achieved with a high power 1550 nm signal and power requirement at 1550 nm is greatly demanding. The second downstream signal uses a 1490 nm wavelength and the upstream digital signal is typically allocated at 1310 nm wavelength.
In FTTP networks, as well in many other applications, a key technology is signal splitting and/or combining. Signal multi/demultiplexing must fulfill very demanding requirements such as, among the other, wide bandwidths and small cross-talk over a wide temperature range (typically from −40° C. to +85° C.). In fact, low cost components, such as light emitter, are used over a wide temperature range, possibly generating a large wavelength drift. Many applications require a rectangular wavelength response in order to maintain a low-loss and wavelength-independent transmission in a passband and a high-level rejection to all wavelengths in a stopband. For example, anticipated telecommunications applications seek a 1.3/1.55 μm WDM filter having a flat and low-loss passband at 1.280-1.335 μm and a −50 dB stopband at 1.525-1.575 μm.
Various devices have been proposed to fill these new, demanding requirements but none are fully satisfactory.
When the separation of two wide bands widely spaced (centered, for example, around λ1=1490 nm and λ2=1550 nm) is needed, micro-optic products based on thin-film technology are currently used, such as for example multilayer thin-film filters in free space optics. Nonetheless, they are undesirable because they cannot be readily integrated and because their fabrication requires high labor cost, mainly due to the operations of alignment of components during assembly, and because of difficulties in coupling light to and from fibers.
Optical splitting components may be classified as the following three types: (1) bulk-type optical splitters; (2) fiber-type optical splitters; and (3) guided-wave type optical splitters.
The bulk-type optical splitters are constructed by arranging microlenses, prisms, interference-film filters, etc., and have little wavelength dependence. Although the bulk-type optical splitters can be put into practical use to some extent, they require a long time and skill for assembly and adjustment, and present some problems with regard to long-term reliability, cost and size.
The fiber-type optical splatters are fabricated using optical fibers as constituent material. Although they exhibit reduced wavelength dependence, the fabrication process requires skill, and is not suitable for mass production because of lack of reproducibility.
In contrast, guided-wave type optical splitters have the advantage that they can be constructed on flat substrates in large quantities through processes such as the photolithography process. Hence, they attract attention as a promising type of splitting component which can be reproduced and integrated as compact parts. Optical devices based on this technology are also referred to as planar lightwave circuits (PLC) devices or integrated optical circuit (IOC) devices.
FIG. 1 is a planar view exemplifying a configuration of a conventional (2×2) guided-wave type optical coupler. In FIG. 1, two optical waveguides 2 and 3 are formed on a substrate 1. A part of the optical waveguide 2 and a part of the optical waveguide 3 are brought into close proximity with each other over a length Lc to form a directional coupler 4. One end of the optical waveguide 2 is a first input port 7 into which an optical signal Pin is launched, and the other end of the optical waveguide 2 is a bar output port 8 from which a bar optical signal Pbar is emitted. Similarly, one end of the optical waveguide 3 is a second input port 5, and the other end of the optical waveguide 3 is a cross output port 6 from which a cross signal Pcross is emitted. The directional coupler 4 may be designed in such a way that an optical signal Pin launched into the first input port 7 is branched into two optical signals Pbar and Pcross to be outputted from ports 8 and 6, respectively.
At a given wavelength, the power coupling ratio C is defined as:
                              C          =                                    P              cross                                                      P                cross                            +                              P                bar                                                    ,                            (        1        )            when the input optical radiation is launched only in the first input port 7.
Generally, the power coupling ratio C of an optical coupler of the type of FIG. 1 may be expressed by the following equation:C=sin2θ(λ)  (2),wherein the coupling angle θ(λ) of the power coupling ratio typically depends on wavelength. In case of a directional coupler 4 of the type of FIG. 1, θ(λ) also depends on the length Lc of the straight coupling region, on the distance between the waveguides at the coupling region over the length Lc, on the shape, width and depth of the waveguides, on the refractive index difference between waveguide core and cladding, on the geometry of the input and output curves 9, etc.
For the purpose of the present invention, only positive coupling angles θ(λ) will be considered.
It is in general possible to express the coupling angle of the optical coupler of FIG. 1 as:θ(λ)=κ(λ)[LC+δL(λ)]  (3),wherein κ(λ) is the wavelength-dependent coupling per unit length in the straight part of the coupler and δL(λ) is an equivalent effective interaction length accounting for the wavelength-dependent coupling contribution of the input and output curves 9.
The optical coupler 4 can act as an optical wavelength splitter, by properly exploiting the wavelength dependence of the power coupling ratio, but its sine-like response does not make it suitable for telecommunication purposes. On the other hand, although the power coupling ratio of the directional coupler 4 can be specified to a desired value at a particular desired wavelength, the wavelength dependence of the coupling ratio presents a problem when the optical coupler is used in a wide wavelength region.
Mach-Zehnder Interferometers (MZI) have been widely employed as optical band splitter/combiner, but they have a sinusoidal response, giving rise to strongly wavelength-dependent transmission and a narrow rejection band.
Other designs have encountered a variety of practical problems.
Article K. Jinguji, N. Takato, Y. Hida, T. Kitoh, and M. Kawachi, “Two-port optical wavelength circuits composed of cascaded Mach-Zehnder interferometers with point-symmetrical configurations,” J. Lightwave Technol., vol. 14, pp. 2301-2310, October 1996, discloses in FIG. 12 a frequency multi/demultiplexer having flattened passbands and stopbands. It is created out of single-stage MZI basic circuits by means of two point-symmetry connection procedures and comprises five directional couplers and, interleaved therebetween, four regions for optical differential delay formed by two wavelength arms with a path difference ΔI. The coupling ratios of the couplers are not functions of wavelength. The path difference of the single-stage MZI is determined from the frequency period of the filter characteristics. The spectral transmittance is periodic for optical frequency, since directional couplers exhibit negligible wavelength dependence. The frequency period is set at 10 GHz.
The frequency multi/demultiplexer described in Jinguji et al. is designed for frequency multi/demultiplexing and has very narrow stopband and passband. In particular, as shown in FIG. 12(b) of Jinguji et al., the stopband and passband are narrower than 10 GHz, which means significantly narrower than 1 nm at typical optical transmission wavelengths (from 850 nm to 1800 nm). The Applicant thus observes that the above device is not suitable to split/combine two optical wavelength bands wider than several nm, such as in 1.49/1.55 μm WDM optical transmission system. Moreover, the multi/demultiplexer described therein is not intended for high density optical device, as shown by the relatively low refractive index difference (0.3%). Moreover, Applicant has determined that the structure disclosed in Jinguji et al. exhibits a limited tolerance to fabrication errors, especially in view of a dense optical integration, as will become clear below.
Monolithic optical waveguide devices are particularly promising because they can perform complex circuit functionalities and because they can be made by mass production integrated circuit techniques. The integration of all the components needed for the full functionality in a single optical integrated circuit may reduce the alignment problem. Moreover, a single integrated chip may allow to a larger extent the automation during the module assembly.